<!DOCTYPE html>
<html lang="en">
<head>
    <meta http-equiv="content-type" content="text/html;charset=UTF-8">
    <title>Title</title>
    <style>
        .c1{
            background-color: red;
            height: 50px;
        }
        @media (min-width: 800px) {
            /*当宽度小于800px时，下面样式生效*/
            .c2{
                background-color: #2459a2;
            }
        }
    </style>
</head>
<body>
<div class="c1 c2"></div>
</body>
</html>